The overall goal of this R21 research proposal is to develop and evaluate the potential of a novel chimeric construct based on Pvs25 as an effective Plasmodium vivax transmission blocking vaccine (TBV). Given the emergence and spread of drug-resistant parasites, and the global malaria eradication agenda, the development of novel tools to control malaria transmission is an essential priority. Plasmodium vivax is the most prevalent of the human malaria parasites outside Africa with an estimated 80% of the cases in South and Southeast Asia and 70% in the Americas. Although Pvs25 vaccine constructs have reached a phase of clinical development, a main concern in the field is its poor immunogenicity. Strategies used to date to enhance the immunogenicity include changes in the delivery system and formulation. However, a clinical trial using a water- in-oil emulsion was halted due to severe systemic adverse events. New methodologies to improve safety and immunogenicity of Pvs25 are required. We have recently shown that the homospecific CD4+ help, provided by the genetic linkage of tandem Plasmodium promiscuous T cell epitopes to a merozoite vaccine candidate, improves efficacy by a direct effect on the quality of the antibody response. We hypothesized that this T helper module (HM) can be used as a carrier molecule to enhance the immunogenicity of Pvs25 and induce a robust transmission blocking immunity. We have genetically fused the synthetic gene encoding HM into a codon optimized synthetic gene encoding Pvs25. The hybrid gene has been successfully expressed in E. coli in a properly folded conformation. The studies proposed aim to (1) test in comparative experiments in mice the effect of a HM on Pvs25 immunogenicity and (2) assess the safety and immunogenicity of the novel chimeric construct in rhesus macaques. Transmission blocking immunity will be tested in vivo using a P. berghei transgenic parasite expressing Pvs25 and in vitro using standard membrane-feeding assays. These studies have the general goal of showing that a HM can be used as a carrier platform for poorly immunogenic malaria antigens. Given the dramatic impact of malaria with increasing attention on the widespread and severe nature of P. vivax, and the urgent need for novel control measures to reduce transmission, our proposal has the potential to serve as the framework for future development of effective multi-stage vaccines. PUBLIC HEALTH RELEVANCE: Malaria caused by P. vivax is a major worldwide health problem with likely annual infections estimated at 132 to 391 million of cases. P. vivax is characterized by its wider geographic distribution than P. falciparum with 2.6 billion of people at risk of infection. P. vivax malaria is resurging and now represents a serious threat in areas where it had been eradicated. The emergence and spread of P. vivax drug resistant strains has brought increased emphasis to the need for alternative prophylactic and therapeutic strategies to control this infection. An effective transmission blocking vaccine is an essential tool for a malaria elimination program.